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Use of break statement causing issue

Asked by TianShu Fan on 10 Apr 2019
Latest activity Answered by Guillaume
on 10 Apr 2019
Hi guys, I was doing a simulation of a simplified stock market, and I wanted to store values for both the first time my investment doubled and the end value of my investment after 10 years. I used an if statement with a break command to store the first time of doubling. But after implementing that, my end value reproduced the exact same values for all my simulation runs.
Here is my code: If I comment out the if statement, my code for storing end value works again...
% Initial Investment v_i
v_i=5000;
% Monthly contribution p
p=200;
% Generate random interest rate r
rmin=-0.05;
rmax=0.05;
% Number of month n
n=120;
v=zeros(n,1);
v(1)=v_i;
% Number of simulations s
s=250;
% Storing V120 data
h=zeros(s,1);
% Storing doubled time data
d=zeros(s,1);
for j=1:s
r=rmin+rand(250,1)*(rmax-rmin);
for i=2:n
% if v(i)>=2*v_i
% d(j)=i;
% break
% end
v(i)=(1+r(i))*v(i-1)+p;
end
h(j)=v(120); %PROBLEM if the above if statement is left uncommented
end
hmax=max(h);
hmin=min(h);
disp(hmax)
disp(hmin)
% plot graph for part b
figure(1)
plot(1:n,v)
xlabel("month")
ylabel("investment amount")
title("month vs investment amount")
print('partb.png', '-dpng')
% plot graph for V120 distribution
figure(2)
histogram(h,s)
xlabel("V120 value")
ylabel("occurence")
title("distribution of terminal value of investment")
print('partc2.png', '-dpng')
% c.iii
havg=mean(h);
disp(havg)
hstd=std(h);
disp(hstd)
% c.iv
figure(3)
histogram(d,s)
xlabel("month")
ylabel("occurence")
title("investment doubling time")
print('partc4.png', '-dpng')
Thank you!

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1 Answer

Answer by Guillaume
on 10 Apr 2019

I'm not sure what you think break does. It completely stops the loop. So, when your code hits break, you stop the i loop and the remaining months never get calculated. You then move to the next j. I don't think that's what you intended.
If you wanted to do something only once in the loop based on some condition, you could use a flag, eg.:
%...
for j=1:s
r=rmin+rand(250,1)*(rmax-rmin);
targetreached = false; %flag
for i=2:n
if v(i)>=2*v_i && ~targetreached
d(j)=i;
targetreached = true;
end
v(i)=(1+r(i))*v(i-1)+p;
end
%...
However, in this case, you really don't need to bother with the test in the loop. You can just get the value at the end of the loop:
for j=1:s
r=rmin+rand(250,1)*(rmax-rmin);
for i=2:n
v(i)=(1+r(i))*v(i-1)+p;
end
d(j) = find(v(i) >= 2*v_i, 1) + 1; %first index of first element whose value is >=2*v_i. That index corresponds to i-1, so add 1.
%...

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